RFR(S): 8172145: C2: anti dependence missed because store hidden by membar

[Doerr, Martin]

Hi,

we found a C2 Compiler bug which leads to incorrect reordering of memory accesses on PPC64 due to missing anti-dependency. Details are described here:
https://bugs.openjdk.java.net/browse/JDK-8172145

Just a remark:
In this case, it would be ok to reorder the load with the membar because the Java Memory Model allows that (normal load followed by volatile load, see [1]). The problem is that the membar hides the path to the store.

The issue can be fixed by this webrev:
http://cr.openjdk.java.net/~mdoerr/8172145_C2_antidep/webrev.00/

This change enables the path which sets LCA=early for the described situation. Then, the load gets scheduled in the same basic block as the membar (just in front of it).

Please review. I will also need a sponsor, please.

Thanks and best regards,
Martin


[1] http://g.oswego.edu/dl/jmm/cookbook.html

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